There has been well known a dispersing apparatus designed to agitate or stir a mixture including a liquid or solid dispersoid and a liquid dispersion medium (the mixture will hereinafter be referred to as “dispersion-material mixture”), at a high speed using a rotary-type agitating device, so as to disperse the dispersoid into the dispersion medium through means of a shearing force to form a dispersed system, such as emulsion (emulsified liquid) or suspension (suspension liquid) (the dispersing apparatus will hereinafter be referred to as “high-speed dispersing apparatus”). In this dispersed-system forming process, a surface-active agent or surfactant is generally added into the dispersion-material mixture to facilitate the dispersion of the dispersoid in the dispersion medium or to stabilize the dispersed system.
In the dispersed-system forming process, a higher dispersity (dispersibility) of the dispersoid in the dispersion medium can be obtained as the surfactant is added at a greater amount, and the agitating device applies a higher shearing force to the dispersion-material mixture. That is, in cases where it is intended to achieve a given dispersibility, a greater addition amount of surfactant allows the intended dispersibility to be achieved by a lower shearing force, and a less addition amount of surfactant raises the need for achieving the intended dispersibility by a higher shearing force.
Generally, in view of the quality of a dispersed system, it is desired to minimize the amount of surfactant to be included in the dispersed system. Moreover, in the dispersed-system forming process using a surfactant, the resulting waste products, such as wastewater, inevitably containing a part of the surfactant involves difficulties in being purified through typical wastewater treating techniques, such as an activated sludge method. Therefore, there is the need for minimizing the addition amount of surfactant in the dispersed-system forming process.
As a measure for meeting this need, it is conceivable to drastically increase the speed of an impeller in an agitating device so as to provide a higher shearing force to be applied to a dispersion-material mixture to reduce the addition amount of surfactant. However, the conventional high-speed dispersing apparatus has a problem that while a shearing force can be increased up to a given level by increasing the speed of the impeller, it will not be appropriately increased beyond the given level despite the increase of the impeller speed. This phenomenon would occur for the reason that when the impeller speed is increased beyond a given threshold value, the dispersion-material mixture contained in a vessel starts rotating integrally with the impeller in its entirety to thereby cause difficulties in the collision between the dispersed-system-material mixture and the inner wall of the vessel.